Ultrasound diagnostic apparatuses are medical image apparatuses for irradiating a subject with an ultrasound wave and obtaining tomographic images (diagnosing images) of a soft tissue based on a reflected wave from each tissue in a living body. Because of their high safety, the ultrasound diagnostic apparatuses are widely used for various diagnoses.
In the case of monitoring a lesion over time with reference to diagnosing images acquired with an ultrasound diagnostic apparatus, how the size of a displayed tissue of the lesion and the shape of a periphery of the lesion changed, or how the luminance of the lesion region changed with respect to the surrounding tissue can be determined by comparing a previously acquired diagnosing image (past diagnosing image) with a newly acquired diagnosing image (present diagnosing image) of a same region in a same subject, so that differential diagnosis of whether the lesion is taking a change for the worse or the better can be implemented.
In order to reliably perform the differential diagnosis, it is necessary to acquire the past diagnosing image and the present diagnosing image with the same image quality (including the same position, direction and luminance). It is required, therefore, that an angle of the ultrasound probe in acquiring the past diagnosing image should be identical to an angle of the ultrasound probe in acquiring the present diagnosing image. The angle of the ultrasound probe should preferably be adjusted easily in a short period of time.
Accordingly, an ultrasound diagnostic apparatus has conventionally been proposed which provides a guidance display so that present position and posture of the probe can coincide with the position and posture of the probe in the past diagnosis (see, for example, Patent Literature 1). The conventional ultrasound diagnostic apparatus is structured to measure spatial position and posture of the probe with use of a magnetic sensor provided on the probe and a magnetism generator placed in a bed and the like, to display a registered probe mark based on the measurement data in the past diagnosis while displaying a present probe mark based on the present coordinate data, and to display proximity or coincidence of the registered coordinates and the present coordinates on the guidance display.
For measurement of the spatial position and posture of the probe in the conventional ultrasound diagnostic apparatus, it is necessary not only to provide a magnetic sensor on the probe but also to place a magnetism generator in a bed and the like, i.e., it is necessary to place a large-scale apparatus.
However, in the ultrasound diagnosis, since the ultrasound probe is used in the state of being in contact with a subject, an operator can sensuously understand the positional relation between the ultrasound probe and the subject and does not need the use of such a large-scale apparatus in most cases. For example, measurement of IMT (Intima-Media Thickness) of the carotid artery in arteriosclerosis diagnosis is one of these cases.
Since diagnosis by IMT measurement is conducted by putting an ultrasound probe on the neck surface in advance, an operator can sensuously understand the positional information on the ultrasound probe and the subject and therefore does not need the use of a large-scale apparatus. What is important in the IMT measurement is the angle information of the ultrasound probe which is brought into contact with the subject.